The present invention is directed to finishing toothed articles, such as gears. Particularly, the present invention discloses a method of finishing gears which results in a significant noise reduction when the gears are rolled in mesh with mating gears.
It is well known in the gear industry that the area of bearing contact between tooth surfaces in mesh should be limited to keep the contact area within the boundaries of the teeth, thus preventing the tooth surfaces from coming into contact at their edges which can lead to tooth damage and/or gear failure.
In order to limit the area of tooth contact, it is necessary to modify a theoretical conjugate tooth flank surface by introducing modifications to limit the contact area under no load or light load to provide insensitivity to things such as gear housing tolerances, inaccuracies in the gear members and assembly, as well as deflections. Thus, instead of the entire tooth surface of mating flanks coming into contact during rolling, as would be the theoretical case with completely conjugate tooth flanks and a drive system having zero deflections and tolerances, mating flanks that have been modified usually contact one another at one point or along a line. Hence, the mating flank surfaces are conjugate only at this point or along the line. Contact is limited to an area of a size such that the contact area will remain within the tooth boundaries despite the effects of actual deflections, tolerances and load.
In bevel gears, there are three mechanisms for creating tooth flank surface modifications that have the intent to locate the tooth bearing contact. These modifications are generally known as xe2x80x9ccrowningxe2x80x9d. Specifically, the three types of crowning are: (1) xe2x80x9clengthwisexe2x80x9d crowning which is a modification along the length (toe-to-heel or face width) of a tooth; (2) xe2x80x9cprofilexe2x80x9d crowning which is a modification in the profile direction (root-to-top) of a tooth; and, (3) xe2x80x9cflank-twistxe2x80x9d crowning which is a twisting of a tooth flank in the length direction (from toe to heel). One or more of the above types of crowning can be applied to a tooth surface but usually all three types of crowning are utilized.
With crowning, however, comes motion error introduced by non-conjugate members rolling in mesh with one another. Generally, as crowning increases, so does the amount of motion error introduced into the mating tooth pair. Increased crowing does protect the teeth from damages of edge contact, however, the accompanying increased amount of introduced motion error prevents smooth rolling of the gears.
With motion error comes noise. Noise is due, to a large extent, to the impact of two mating teeth coming into mesh. It is known that as a pair of mating teeth with a parabolic motion graph roll in mesh, there is a linear decrease in angular velocity of the teeth of one member relative to the teeth of the other member. As such, relative angular acceleration has a constant negative value. However, as contact changes from the actual pair of teeth in mesh to the following pair coming into mesh, there is an instant increase in relative velocity, since the initial relative velocity of the following pair is greater than the final relative velocity of the actual pair. Given this sudden increase in velocity, there is likewise an effective momentary infinite increase in relative angular acceleration which physically is an impulse (i.e. a noise) that reflects the impact the following pair of teeth causes at the moment of first contact. This noise is repeated for each pair of teeth as they first come into contact. The result of these contacts is an audible noise of the tooth mesh frequency and/or multiples thereof.
One method that has been used to address the problem of gear noise is lapping. The highest removal of material in lapping takes place at the instant of impact because of the peak torque between the two mating members. In other words, the material that leads to disturbances will be removed during lapping. However, surface studies on lapped gearsets have shown that some abrasive particles from the lapping compound attach themselves to the tooth flank which means a continuous xe2x80x9clight lappingxe2x80x9d takes place at all times when a gear set is in operation. Furthermore, the lapping compound particles tend to move from the tooth surface into the oil which lubricates the gear set thus amplifying the negative effect even more.
A proposal for reducing gear noise by introducing a fourth-order crowning along the path of contact is set forth in Stadtfeld, Handbook of Bevel and Hypoid Gears, Rochester Institute of Technology, Rochester, N.Y., 1992, pp. 135-139. The disadvantage associated with this approach is that it is effective under high load conditions but not under noise critical low load conditions.
Still another theoretical proposal to reduce gear noise is described in Litvin et al., xe2x80x9cMethod for Generation of Spiral Bevel Gears With Conjugate Gear Tooth Surfacesxe2x80x9d, Journal of Mechanisms, Transmission, and Automation in Design, Vol. 109, June 1987, pp. 163-170. In this procedure, crowning is introduced along the lines of contact. However, this process is ineffective in reducing noise.
It is an object of the present invention to provide a process for machining toothed articles which greatly reduces gear noise arising from impact of teeth as they enter into mesh.
It is another object of the present invention to provide a gear having at least one tooth surface made in accordance with the above process.
The present invention is directed to a method of machining at least one tooth flank of a gear with a finishing tool. The method comprises rotating the tool, such as a grinding tool, and bringing the tool and the tooth flank into contact. Relative movement between the tool and the gear is provided to traverse the tool across the tooth flank along a path whereby the path produces a tooth flank geometry of a form which, when brought into mesh with a mating tooth flank, under no load or light load, to form a tooth pair, provides a motion graph curve that intersects, at least two times, a motion graph curve of at least one of an immediately preceding tooth pair and an immediately following tooth pair.
The motion graph curve of the tooth pair may describe a fourth, or higher, even order function and is preferably of a shape comprising two maxima separated by two inflection points. The motion graph curve of the tooth pair describes contact between respective tooth flanks, under no load or light load, of said tooth pair from an initial entrance into mesh to a final exit from mesh as being over a gear rotation amount greater than 1.0 pitch and preferably, about 1.5 pitch to about 3.0 pitch.